In recent years, the relatively heavy metal blades and vanes of fluid flow machines such as gas turbine engine fans and compressors have been replaced with lighter composite materials. These composite blades and vanes are fabricated to have high strength, and are made from plies comprising elongated fibers in a light weight matrix.
Over the years the term composite has had several meanings regarding the use of two or more materials having different properties. In the aerospace industry, the term composite has come to be defined as a material containing a reinforcement such as fibers or particles supported in a binder or matrix material. The composite blades and airfoils of the present invention are preferably of the non-metallic type made of a material containing a fiber such as a carbonaceous, silica, metal, metal oxide, or ceramic fiber embedded in a resin material such as epoxy, PMR-15, BMI, PEEK, etc. Of particular use are unidirectional fiber-reinforced prepreg composite sheets, laid up in a predetermined sequence and formed into a part shape, and cured via an autoclaving process or press molding to form a light weight, stiff, relatively homogeneous article having laminates within.
Many types of foreign objects may be entrained in the inlet of a gas turbine engine, ranging from large birds, such as sea gulls, to hailstones, sand and rain. Damage from foreign objects, referred to as foreign object damage (FOD), takes two forms. Smaller objects can erode the blade material and degrade the performance of the fan and engine. Impact by larger objects may fracture or pierce the blades. Portions of an impacted blade can be torn loose and cause extensive secondary damage to the downstream blades and other engine components.
In this regard, the consequences of foreign object damage are greatest in the low pressure compressors and fans of high bypass gas turbine engines. However, these components offer the greatest potential in weight reduction due to their size. For example, tip diameters on fan blades of high thrust jet engines are as great as ten feet, and have spans in the order of two or more feet. Many developments have been made to prevent composite fan blade failures such as a metallic leading edge protection strip which also helps provide erosion protection for the fan blade.
One particular FOD-related failure mode of composite fan blades is bending and delamination of the blade when it is struck by a heavy object such as a bird, particularly in a region near the radially outward blade tip. This, in turn, can result in secondary engine damage as the blade fragments, including the leading edge protection strip, are ingested through the engine.
Thus, it has become highly desirable to develop light weight composite blades. Of particular importance are long span fan blades made of light weight non-metallic materials for a high bypass ratio gas turbine engines which resist delamination due to bending induced by foreign object impact into the blade.
One such light weight composite fan blade is set forth in U.S. Pat. No. 5,375,978. This patent sets forth sequencing of plies to provide a large composite airfoil having a high degree of twist. Groups of plies are arranged in order by span height, shortest to tallest starting at the centerplane. Each group of plies has four laminations arranged in an angular sequence of 0°, +Φ, 0°, −Φ° where Φ° is a predetermined angle measured from 0°. The progression of the groups is broken by at least one group of relatively tall laminations. The groups also may be arranged so that no two adjacent groups are in order of span height progression. The result is a light weight blade with a high degree of twist laid up generally along the centerplane so that a shear plane is not created where radially outer edges of the laminations end.
One development to prevent delamination is fabricating a fan or compressor blade by laying up and bonding together a plurality of unidirectional prepreg plies. Ideally, the unidirectional fibers of at least a portion of the plies are skewed, in a chordwise direction, forward and aft of a non-radial blade axis, thus forming a biased lay-up with the blade center of twist biased forward or aft of the blade radial axis. This significantly increases the torsional frequency of the blade.
It is well known that the blade includes a root section, where the blade is attached to a rotating disk, and an airfoil section extending into the air flow path. The number of plies may run on the order of one thousand in the root area of the blade. Ply thickness is usually determined by the material to be used and is on the order of 4-6 mils per ply. The span height, width, and shape depends, at least in part, on the shape and contour of the blade. Typically, three-dimensional computer aided design (CAD) systems are used to design ply shapes and span heights. The CAD system also determines, based on pre-determined criteria, the precise order of lay-up of the plies to achieve the final part form while maintaining optimum strength characteristics.
To achieve desired strength characteristics, the fibers within each ply should remain unidirectional. The long spans and high degrees of twist characteristic of aerodynamic blades in modern high bypass ratio turbofan engines do not allow for a single piece of material to be used in each ply and still maintain fibers which are unidirectional within the ply. Because of complex airfoil geometries, the CAD system may design a lay-up comprising a plurality of ply sections laid up adjacent to one another to form a single ply layer.
Many of these complex geometric parts require hand-lay-up. In order for the finished part to maintain the close tolerances required by the complex geometries, it is critical that each ply section be placed precisely in its CAD system predetermined location during the hand-lay-up process. To aid in this precise placement, laser projection systems, often referred to as an “Optical Lay-up Template” (OLT) utilize 3-D data sets calculated by the CAD system to accurately identify placement locations on a work surface, for example, a lay-up tool or a ply layer. These rapidly scanning laser systems move a laser beam from location to location with sufficient speed to appear as a continuous line. A plurality of lines are used to precisely define the borders in which to place the ply section. The exact sequence of ply section placement, as determined by the CAD system is programmed into the OLT.
One problem encountered is that a prepreg ply section can be deformed during hand lay-up. While a unidirectional prepreg ply section is resistant to distortion along the fiber axis, it can be easily distorted by forces directed at an angle to the fiber axis, particularly when directed 90 degrees to the fiber axis. Such transverse distortion forces inadvertently applied during the hand lay-up process may “stretch” the ply, causing the laid-up ply section to extend beyond one or more of the predetermined CAD system borders, adversely affecting final part form and/or creating non-unidirectional fiber patterns, thereby adversely affecting final part strength.
The present invention provides a method to reduce or eliminate distorted ply sections inadvertently produced during the hand lay-up process and provides other related advantages.